This invention relates to valves installed in tubing strings. More particularly, but not by way of limitation, this invention relates to a combination two-way check valve and back pressure valve which in one embodiment performs as a two-way check valve, and in another embodiment performs as a back pressure valve.
A typical oil and gas well completion comprises a well bore that intersects a reservoir. A tubing string provides for a conduit so that the produced oil, gas and water may flow to the surface. Once at the surface, a series of surface valves (also known as the christmas tree) control the flow of the oil, water and gas.
During the life of the reservoir, the operator may deem it advisable to workover the well, which may require reworking the present zone, or alternatively, recompleting to another zone. In the course of workover operations, the christmas tree will be removed, and blow-out preventers will be joined to the tubing string via the tubing head and tubing hanger. In order to perform this operation, it is necessary to first install either a check valve or back pressure valve.
The check valve usually employed is well known to those of ordinary skill in the art. For instance, Cameron sells a "H" & "HM" Two-Way Check Valve. The valve is installed by using a lubricator, also well known in the art, such as the P-10 lubricator sold by Lubricators International. The advantage of using a check valve is that once in place, the blow-out preventer stack can be pressure tested for any leaks in the stack, manifold, etc. The two-way check valve can then be removed by the lubricator.
The typical back-pressure valve, which may be purchased from Cameron Iron Works, Inc. and known as the "H" Back Pressure Valve, may be used after the workover procedure. Thus, a back-pressure valve will be placed in the tubing hanger, the blow-out preventer stack will be disassembled, and the christmas tree will be put back in place. If pressure from the formation has built back up, fluid can be pumped through the back-pressure valve, and down the tubing string thereby creating a balanced hydrostatic head (also known as killing the well).
Prior art two-way check valves contain certain inherent deficiencies. For instance, when testing the Blow-Out Preventer's, if pressure has built up from the formation, it will be necessary to rig down the Blow-out Preventer's stack, re-install the christmas tree, remove the two-way check valve, locate in its place the traditional back pressure valve, and pump fluid down the tubing string in order to equalize the hydrostatic pressure i.e. kill the well.
Of course, it is not possible to first install the back pressure valve because of its inability to resist pressure from the top side, thereby precluding a testing of the Blow-Out Preventers. However, if the two way check valve is used, but is required to be replaced because of pressure build-up, such an operation is time consuming and very costly.
Therefore, it is an object of this invention to provide a combination two-way check valve that can be converted to a back-pressure valve by applying a pressure force to the two way check valve at the option of the operator. Another object of the invention is to combine the use of two separate tools into one which is more simple, and cost effective apparatus. Yet another object of the invention is to eliminate the procedure of changing out a two way check valve in order to locate a back pressure valve to equalize well pressure.